A base station schedules one of a plurality of mobile terminals based on an expected sir of an effective traffic channel associated with a non-scheduled mobile terminal. The expected sir is generated by computing the expected sir of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled. A base station then schedules transmissions to the plurality of mobile terminals based on the computed expected sir. The expected sir may be computed so as to compensate for mismatch between the hypothesized traffic channel and a pilot channel associated with the non-scheduled mobile terminal. Alternatively, the expected sir may be directly computed based on an estimate of the pre-filter of the hypothesized traffic channel.
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1. A method of estimating an expected signal-to-interference ratio (sir) of an effective traffic channel for a non-scheduled mobile terminal comprising:
computing the expected sir of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled.
92. A circuit to implement a process to estimate an expected signal-to-interference ratio (sir) of an effective traffic channel for a non-scheduled mobile terminal, the circuit comprising:
an sir processor to compute the expected sir of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled.
84. A computer readable media for storing a set of instructions to estimate an expected signal-to-interference ratio (sir) of an effective traffic channel for a non-scheduled mobile terminal, the set of instructions comprising:
instructions to compute the expected sir of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled.
60. A non-scheduled mobile terminal responsible for assisting a base station in scheduling one of a plurality of mobile terminals comprising:
a receiver to receive a pilot signal from the base station; and
a signal-to-interference ratio (sir) calculator to determine an expected sir of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled.
32. A method of scheduling one of a plurality of mobile terminals in a wireless communication system comprising:
computing an expected signal-to-interference ratio (sir) of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled; and
scheduling one of the plurality of mobile terminals based on the expected sir of the hypothesized traffic channel.
40. A base station responsible for scheduling transmissions to one of a plurality of mobile terminals in a wireless network, the base station comprising:
a transmitter to transmit data to a plurality of mobile terminals over a time-multiplexed data channel; and
a scheduler to schedule transmissions to one of the plurality of mobile terminals based on expected signal-to-interference ratios (sir)s, wherein the expected SIRs for non-scheduled mobile terminals are based on hypothesized traffic channels with pre-filters adapted to the non-scheduled mobile terminals that would result if the non-scheduled mobile terminals were scheduled.
2. The method of
computing the expected sir of the hypothesized traffic channel so as to compensate for mismatch between the hypothesized traffic channel and a pilot channel associated with the non-scheduled mobile terminal,
wherein said mismatch is due to the pre-filter that would result if the non-scheduled mobile terminal was scheduled.
3. The method of
measuring a pilot sir of the pilot channel associated with the non-scheduled mobile terminal; and
applying a correction factor to the measured pilot sir to compensate for the mismatch between the hypothesized traffic channel and the pilot channel associated with the non-scheduled mobile terminal.
4. The method of
5. The method of
6. The method of
7. The method of
storing the correction factors for different values of pilot sir in a look-up table; and
selecting the correction factor corresponding to the measured pilot sir from the look-up table.
8. The method of
9. The method of
storing the correction factors for different values of signal delay spread in a look-up table; and
selecting the correction factor corresponding to a current signal delay spread from the look-up table.
10. The method of
11. The method of
12. The method of
13. The method of
storing the correction factors for different values of transmit power ratio in a look-up table; and
selecting the correction factor corresponding to a current transmit power ratio from the look-up table.
14. The method of
15. The method of
16. The method of
17. The method of
storing the correction factors for different combinations of pilot sir and transmit power ratio in a look-up table; and
selecting the correction factor corresponding to the measured pilot sir and a current transmit power ratio from the look-up table.
18. The method of
19. The method of
storing the correction factors for different combinations of pilot sir and signal delay spread in a look-up table; and
selecting the correction factor corresponding to the measured pilot sir and a current signal delay spread from the look-up table.
20. The method of
21. The method of
storing the correction factors for different combinations of pilot sir, transmit power ratio, and signal delay spread in a look-up table; and
selecting the correction factor corresponding to the measured pilot sir, a current transmit power ratio, and a current signal delay spread from the look-up table.
22. The method of
estimating the pre-filter of the hypothesized traffic channel; and
computing the expected sir based on the estimated pre-filter.
23. The method of
24. The method of
25. The method of
26. The method of
where αs represents the fraction of the total transmit power allocated to the pre-filtered traffic channel, ET represents the total transmit power, Îo represents the estimated noise level associated with the hypothesized traffic channel corresponding to the non-scheduled mobile terminal, hn represents a channel gain matrix corresponding to the non-scheduled mobile terminal, and Rn represents an impairment covariance matrix corresponding to the non-scheduled mobile terminal.
27. The method of
estimating an interference level that would result if the non-scheduled mobile terminal was scheduled; and
processing the interference level to estimate the noise level.
28. The method of
processing the received pilot signals to reconstruct the pilot signal; and
subtracting the reconstructed pilot signal from the received pilot signals to estimate the interference level.
29. The method of
generating a set of K interference levels estimated over K frames of a received signal; and
selecting the minimum interference level within the set of interference levels as the estimated noise level.
30. The method of
31. The method of
33. The method of
selecting a preset noise level;
computing the expected sir for each of the plurality of non-scheduled mobile terminals based on the preset noise level; and
comparing the expected SIRs to generate a set of relative sir estimates.
34. The method of
35. The method of
36. The method of
37. The method of
measuring a pilot sir of the pilot channel associated with the non-scheduled mobile terminal; and
generating a corrected sir by applying a correction factor to the measured pilot sir to compensate for the mismatch between the hypothesized traffic channel and the pilot channel associated with the non-scheduled mobile terminal.
38. The method of
39. The method of
estimating the pre-filter of the hypothesized traffic channel; and
computing the expected sir based on the estimated pre-filter.
41. The base station of
42. The base station of
43. The base station of
44. The base station of
45. The base station of
46. The base station of
47. The base station of
48. The receiver of
49. The base station of
50. The base station of
51. The base station of
52. The base station of
53. The base station of
54. The base station of
55. The base station of
56. The base station of
57. The base station of
58. The base station of
59. The base station of
61. The non-scheduled mobile terminal of
62. The non-scheduled mobile terminal of
63. The non-scheduled mobile terminal of
64. The non-scheduled mobile terminal of
65. The non-scheduled mobile terminal of
66. The non-scheduled mobile terminal of
67. The non-scheduled mobile terminal of
68. The non-scheduled mobile terminal of
69. The non-scheduled mobile terminal of
70. The non-scheduled mobile terminal of
71. The non-scheduled mobile terminal of
72. The non-scheduled mobile terminal of
73. The non-scheduled mobile terminal of
74. The non-scheduled mobile terminal of
75. The non-scheduled mobile terminal of
76. The non-scheduled mobile terminal of
77. The non-scheduled mobile terminal of
78. The non-scheduled mobile terminal of
79. The non-scheduled mobile terminal of
80. The non-scheduled mobile terminal of
where αs represents the fraction of the total transmit power allocated to the pre-filtered traffic channel, ET represents the total transmit power, Îo represents the estimated noise level associated with the hypothesized traffic channel corresponding to the non-scheduled mobile terminal, hn represents a channel gain matrix corresponding to the non-scheduled mobile terminal, and Rn represents an impairment covariance matrix corresponding to the non-scheduled mobile terminal.
81. The non-scheduled mobile terminal of
82. The non-scheduled mobile terminal of
a reconstructor to reconstruct a pilot signal from the received pilot signal; and
a combiner to subtract the reconstructed pilot signal from the received pilot signal to estimate the interference level.
83. The non-scheduled mobile terminal of
85. The computer readable media of
86. The computer readable media of
instructions to measure a pilot sir of the pilot channel associated with the non-scheduled mobile terminal; and
instructions to apply a correction factor to the measured pilot sir to compensate for the mismatch between the hypothesized traffic channel and the pilot channel associated with the non-scheduled mobile terminal.
87. The computer readable media of
instructions to estimate the pre-filter of the hypothesized traffic channel; and
instructions to compute the expected sir based on the estimated pre-filter.
88. The computer readable media of
89. The computer readable media of
90. The computer readable media of
91. The computer readable media of
93. The circuit of
94. The circuit of
95. The circuit of
96. The circuit of
97. The circuit of
98. The circuit of
99. The circuit of
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The present invention relates generally to scheduling mobile terminals on a shared high-speed multi-path propagation channel in a wireless communication system and more particularly to a method for computing signal-to-interference (SIR) estimates for use in making scheduling decisions.
In conventional CDMA systems, a base station (BS) transmits signals to a plurality of mobile terminals simultaneously on a multi-path propagation traffic channel. In the high-speed downlink shared channel (HS-DSCH) mode of wideband code division multiple access (W-CDMA) multi-path propagation, packet transmissions are time-multiplexed and transmitted at the full power available to the BS, but with data rates and slot lengths that vary depending on channel conditions. Thus, the BS transmits to only one mobile terminal at a time.
For the HS-DSCH mode, a scheduler at the BS schedules the multi-path propagation transmission to mobile terminals. The scheduler determines which mobile terminal to serve at any given time. Further, the scheduler determines the data rate for the multi-path propagation transmission and the length of the multi-path propagation transmission. There are many different approaches to scheduling for the HS-DSCH mode, each of which serves different objectives. Perhaps the simplest is round-robin scheduling where each mobile terminal is scheduled in turn to receive multi-path propagation transmission. Other scheduling approaches include maximum C/I (carrier to interference) scheduling or proportionally fair scheduling. The maximum C/I scheduling approach schedules the mobile terminal with the maximum C/I ratio to maximize data throughput. The proportionally fair scheduling approach attempts to be more evenhanded by maintaining the effective data transmission rate for all mobile terminals in the same proportion to the scheduled mobile terminal's maximum achieved rate.
Most scheduling approaches require knowledge of the SIR (signal-to-interference ratio) or SINR (signal to interference plus noise ratio) corresponding to the traffic channel of each mobile terminal being scheduled. The BS obtains SIR estimates from the mobile terminals being scheduled, or calculates the SIR from signal strength measurements made by the mobile terminals and transmitted to the BS. The mobile terminal that is currently scheduled, referred to herein as the scheduled mobile terminal, despreads the traffic channel, despreads the pilot channel, estimates the channel from the pilot channel, computes the traffic channel SIR using the channel estimates and the despread traffic channel, and sends the estimated traffic channel SIR and/or some other SIR-based information, i.e., a channel quality indicator (CQI) to the BS. The mobile terminals that are not currently scheduled, referred to herein as the non-scheduled mobile terminals, measure the received signal strength on the forward pilot channel, estimate the SIR from the pilot strength measurements, and send the estimated pilot SIRs to the BS. Because the transmit powers on the HS-DSCH are typically much larger than the pilot transmit power, the pilot SIR is scaled to obtain an estimate of the traffic channel SIR. Scaling the pilot SIR to estimate the traffic channel SIR produces reasonably accurate estimates when the pilot and traffic signals travel through the same effective channel.
The present invention comprises a method and apparatus for scheduling one of a plurality of mobile terminals, including currently scheduled and non-scheduled mobile terminals, in a wireless communication system based on an expected SIR of an effective traffic channel associated with a non-scheduled mobile terminal. According to the present invention, either the base station or the non-scheduled mobile terminal estimates the expected SIR of an effective traffic channel for each non-scheduled mobile terminal by computing the expected SIR of a hypothesized traffic channel with a pre-filter adapted to the non-scheduled mobile terminal that would result if the non-scheduled mobile terminal was scheduled. A base station then schedules one of the plurality of mobile terminals in the wireless system based on the expected SIR from the non-scheduled mobile terminals and the scheduled mobile terminal.
In an exemplary embodiment, either the base station or the mobile terminal computes the expected SIR of the hypothesized traffic channel so as to compensate for mismatch between the hypothesized traffic channel and a pilot channel associated with the non-scheduled mobile terminal. The mismatch in this embodiment is at least partially attributed to the pre-filter associated with the effective traffic channel of the non-scheduled mobile terminal.
In an alternate embodiment, the base station or the mobile terminal computes the expected SIR of the hypothesized traffic channel by estimating the pre-filter of the hypothesized traffic channel that would result if the non-scheduled mobile terminal was scheduled, and computing the expected SIR based on the estimated pre-filter.
Transmitter 20 employs transmit diversity to transmit a signal s(t) intended for scheduled mobile terminal 30. In the illustrated embodiment, transmitter 20 is configured for the HS-DSCH mode of a W-CDMA system, where a high-speed multi-path propagation channel is shared by time-multiplexing a plurality of mobile terminals 30, 40, as described above. Transmitter 20 also transmits signals, such as pilot channel, associated dedicated physical channel (ADPCH), and overhead channel signals, represented herein by {d1(t), d2(t), . . . dM(t)}, to the scheduled mobile terminal 30 and the non-scheduled mobile terminal 40.
The transmitter 20 includes common filter 22, M channel filters 24, M summers 26, and M antennas 28. Common filter 22 pre-filters s(t) such that the total energy transmitted from all antennas 28 is constant. Each channel filter 24 is matched to the mth multi-path propagation channel between the mth transmit antenna 28 and the receive antenna of the scheduled mobile terminal 30. As such, each channel filter 24 pre-filters s(t) to compensate for the effects of the multi-path propagation channel between the mth antenna 28 and the scheduled mobile terminal 30. Summer 26 combines signal d(t) with the pre-filtered signal s(t). The combined signal is transmitted to mobile terminals 30, 40 via antennas 28.
The total transmit energy emitted by transmitter 20 is divided between s(t) and signals d1(t), d2(t), . . . dM(t) according to predetermined power ratios. For example, a traffic power ratio, represented by αs, may represent the fraction of the total transmitted energy allocated to s(t). The remaining energy represents the transmitted energy allocated to signals {d1(t), d2(t), . . . dM(t)}. As such, a power ratio, represented by αd=1−αs, represents the fraction of the total transmitted energy allocated to signals {d1(t), d2(t), . . . dM(t)}. Exemplary power ratios to total transmit energy may be αs=0.7 and αd=0.3.
Further, a pilot power ratio, represented by αp, may be defined as the fraction of the total energy allocated to the pilot signal on the mth antenna 28. An exemplary pilot power ratio to total transmit energy may be αp=0.1/M, which assumes that 10% of the total transmit energy is allocated to the pilot channel signals, where the pilot transmit energy is divided evenly between the M transmit antennas 28. In conventional wireless communication systems, the traffic power ratio divided by the pilot power ratio, referred to herein as the traffic-to-pilot ratio, is used to estimate the SIR of the HS-DSCH when the mobile terminal is not currently scheduled. The traffic channel SIR of a non-scheduled mobile terminal is determined by scaling the measured pilot SIR based on the traffic-to-pilot ratio, αs/αp.
As seen in
The effective channels are determined as follows. The signal received at the scheduled mobile terminal 30 from the mth transmit antenna may be represented by s(t)*heff,0(t)+dm(t)*g0m(t), and the signal received at the non-scheduled mobile terminal 40 may be represented by s(t)*heff,n(t)+dm(t)*gnm(t). The effective traffic channel associated with the scheduled mobile terminal 30, heff,0(t), and with a non-scheduled mobile terminal 40, heff,n(t), is given by Equations 1a and 1b, respectively,
where hw(t) represents the filter function associated with common filter 22, hm(t) represents the filter function associated with the mth channel filter 24, g0m(t) represents the mth multi-path propagation channel between transmitter 20 and the scheduled mobile terminal 30, and gnm(t) represents the mth multi-path propagation channel between transmitter 20 and a non-scheduled mobile terminal 40. Because hm(t) compensates for the mth multi-path propagation channel between transmitter 20 and the scheduled mobile terminal 30, hm(t)=g*0m(−t). Converting Equation 1a to the frequency domain provides the frequency response of the effective traffic channel for the scheduled mobile terminal 30, which is given by:
Contrastingly, the frequency response of the effective traffic channel for the non-scheduled mobile terminal 40 is given by:
Note that the fixed filter Hw(ω) is implicitly included in Equations 2 and 3. In this case,
As shown by Equations 2 and 3, the effective channel for the scheduled mobile terminal 30 depends only on the effective multi-path propagation channel for the scheduled mobile terminal 30, while the effective channel for the non-scheduled mobile terminal 40 depends on the multi-path propagation channel for both the scheduled and non-scheduled mobile terminals 30, 40. In contrast, the pilot signal received by the scheduled mobile terminal 30 from the mth transmit antenna traverses the channel g0m(t). As a result, a mismatch occurs between the SIR measured on the pilot channel and the SIR on the traffic channel. While conventional SIR estimation methods based on scaling the SIR of the pilot channel will compensate for the power mismatch, these methods do not address the additional mismatch caused by pre-filters 22, 24.
To make scheduling decisions, the base station would like to know the expected SIR of the non-scheduled mobile terminal 40 as if it was scheduled. By analogy to Equation 2, the frequency response of the effective traffic channel of the non-scheduled mobile terminal 40 as if it was scheduled is given by:
As shown in Equation 4, this differs from the effective channel of the non-scheduled mobile terminal 40. The difference further adds to the mismatch between the measured pilot SIR and the traffic channel SIR. As a result of this channel mismatch, the SIR measured at a non-scheduled mobile terminal 40 differs from the SIR that would be measured at the non-scheduled mobile terminal 40 if the non-scheduled mobile terminal 40 was scheduled.
To better appreciate how the measured pilot channel SIR differs from the traffic channel SIR that would be experienced if the non-scheduled mobile terminal 40 was scheduled, consider the following mathematical model. Assume that SIRtrue,n represents the “true” SIR for a non-scheduled mobile terminal 40 as if the non-scheduled mobile terminal 40 was scheduled, and SIRmeas,n represents the measured SIR on the pilot channels for the non-scheduled mobile terminal 40. Further, for simplicity, assume that only one code is used on the HS-DSCH. (Note that for the case of multi-code, the true SIR simply scales with the number of codes used on the HS-DSCH). The average received energy per symbol due to the single code is given by αsET, where ET represents the total received signal energy. The remainder of the received energy is due to the pilots, ADPCHs, and overhead channels, and is given by αdET.
Assuming that mobile terminals 30, 40 use a G-RAKE receiver, let Q represent the total number of fingers used in the G-RAKE receiver, and let q index the fingers. As shown in Equation 4, the effective channel for the non-scheduled mobile terminal 40 as if the non-scheduled mobile terminal 40 was scheduled is denoted
and is given by the inverse Fourier transform of
which depends only on the multi-path propagation channels {gnm(t)}M=1M for the non-scheduled mobile terminal 40. Let P represent the number of taps of the mth multi-path propagation channel gnm(t) for the non-scheduled mobile terminal 40, and let p index each of these taps. The tap gains and delays are denoted gnmp and τnmp, respectively. Further, let L be the total number of taps of the effective channel
for the non-scheduled mobile terminal 40, and let l index each of the taps. The tap gains and delays of the effective channel are denoted hnl and τnl, respectively.
The despread vector containing the despread value for each of the RAKE fingers of the G-RAKE receiver is given by:
yn(i)=√{square root over (αsET)}hnc(i)+zn(i), (Eq. 5)
where c(i) is the symbol of interest during the ith signaling interval, hn is a channel gain vector, and zn(i) is an impairment vector. The qth component of the channel gain vector hn is given by:
where x(τ) is the autocorrelation function of the chip pulse shape and τq is the delay of the qth finger of the G-RAKE receiver.
The impairment vector zn(i) includes (1) inter-symbol interference (ISI) on the HS-DSCH, (2) interference from the non-traffic channel signals associated with the M transmit antennas, and (3) noise plus other-cell interference, which is typically modeled as white noise. An impairment covariance matrix, Rz,n=E└zn(i)znH(i)┘, may be determined by considering the channel definitions introduced in
Equation 7 includes three component matrices, Rs, Rd, and Ro that correspond, respectively, to the three different components of the impairment vector zn(i) discussed above. The (q1, q2)th element of Rs is given by
the (q1, q2)th element of Rd is given by
x(jT+τq1−τnmp1−uTc)x*(jT+τq2−τnmp2−uTc)[1−δ(u)δ(j)],and
the (q1, q2)th element of Ro is given by
{Ro}q1,q2=x(τq1−τq2) (Eq. 10)
where SF is the spreading factor, T is the symbol period, and Tc=T/SF is the chip period. As shown in Equation 8, Rs is a function of the effective channel, which includes the pre-filters 22, 24 designed for the non-scheduled mobile terminal 40 as if the non-scheduled mobile terminal 40 was scheduled. Rd is a function of the multi-path propagation channels themselves, as shown in Equation 9.
The weight vector for the G-RAKE receiver is given by wn=Rz,n−1hn. Applying the weight vector to the despread vector yn(i) gives the decision statistic
Yn(i)=wnHyn(i)=√{square root over (αsET)}wnHhnc(i)+wnHzn(i). (Eq. 11)
From this, the true SIR of the non-scheduled mobile terminal 40 as if the non-scheduled mobile terminal 40 was scheduled, SIRtrue,n is given by:
Equation 12 emphasizes the dependence of SIRtrue,n on the input signal-to-noise ratio (SNR), ET/Io. For small input SNRs, Rn≈Ro, and SIRtrue,n is directly proportional to ET/Io. Consequently, SIRtrue,n increases linearly with ET/Io. For very large input SNRs,
Now that SIRtrue,n has been defined, an expression for the measured SIR on the pilot channels for the non-scheduled mobile terminal 40, SIRmeas,n, is derived for comparison purposes. The measured SIR represents the sum of the measured pilot SIR for each of m∈{1, M} pilot channels, as shown in Equation 13.
To measure the SIR on the pilot channel transmitted from the mth antenna, the spreading code on the channel resulting in the length-Q despread vector is correlated in the G-RAKE receiver, which results in
ynm(i)=√{square root over (αpET)}gnmcm(i)+znm(i), (Eq. 14)
where cm(i) is the pilot symbol of interest transmitted from the mth antenna during the ith signaling interval, and gmn is a channel gain vector with qth component given by
Note that this is different from the despread vector for the HS-DSCH discussed above (see Equation 5) because the channel gain vector of Equation 14 is a function of the multi-path propagation channels gnm(t) rather than the effective channel
which includes the pre-filters 22, 24. This is one reason for the mismatch between SIRmeas,n and SIRtrue,n.
The impairment vector, znm(i) is also different as it includes (1) interference from the HS-DSCH with pre-filters designed for the scheduled mobile terminal 30, (2) ISI on the mth pilot channel, (3) interference from the pilot, ADPCHs, and overhead channels associated with the other antennas, and (4) noise plus other-cell interference (typically modeled as white noise). The resulting covariance matrix is given by
The first component of Equation 16 contains the interference from the HS-DSCH with pre-filters designed for the scheduled mobile terminal 30. The second and third components are identical to Equation 7.
The covariance matrix of Equation 16 appears similar to the covariance matrix of Equation 7. However,
defined in Equation 4. This difference is another reason for the mismatch between SIRmeas,n and SIRtrue,n.
As discussed above, Heff,n(ω) is a function of the multi-path propagation channels of both the scheduled and non-scheduled mobile terminals. Denoting
which has the same form as Equation 8, except the channel tap gains and delays are different.
The weight vector for the G-RAKE receiver for the mth pilot channel is given by wnm=
Ynm(i)=wnmHynm(i)=√{square root over (αpET)}wnmHhnmc(i)+wnmHznm(i). (Eq. 18)
From this the measured SIR of the non-scheduled mobile terminal 40 on the mth pilot channel is given by
As discussed above and shown in Equations 12 and 19, there is a mismatch between the true SIR (SIRtrue,n) and the measured SIR (SIRmeas,n). Equation 20 provides a comparison of Equation 12 and Equation 19 that better illustrates this mismatch.
As shown by Equation 20, SIRmeas,n differs from SIRtrue,n by more than the simple scaling factor αs/αp. As a result, the simple scaling factor associated with the power ratios will not reliably compensate for the mismatch caused by pre-filters 22, 24. In other words, the effective channel mismatch between the pilot channel of the non-scheduled mobile terminal 40 and the effective traffic channel that would result if the non-scheduled mobile terminal 40 was scheduled renders the simple scaling factor technique of the conventional systems insufficient for systems that pre-filter traffic channel signals.
The present invention addresses the SIR mismatch problem in the non-scheduled mobile terminals 40 by generating an expected SIR of a hypothesized effective traffic channel of the non-scheduled mobile terminal 40 that would have resulted if the non-scheduled mobile terminal 40 was scheduled. While the following discussions focus on the non-scheduled mobile terminal, it will be understood that because the scheduled mobile terminal also encounters a mismatch problem between the traffic channel and the measured pilot channel, the present invention may also be applied to the scheduled mobile terminal.
The present invention may be implemented in any wireless communication system, such as the exemplary wireless communication system 100 shown in
Transceiver 112 further includes a receiver 140 that receives communication signals from mobile terminals 150, 160 via antenna 114. Receiver 140 also receives scheduling information, i.e., SIR estimates (SIRest) from the scheduled mobile terminals 150 and the non-scheduled mobile terminal 160, a representation or mapping of the SIRest, such as a channel quality indicator (CQI), and/or, in some cases, SIR variables from one or more non-scheduled mobile terminals 160. When SIR variables are provided to base station 110, the receiver provides the SIR variables to an SIR processor 118 in base station 110 to generate the expected SIR for the mobile terminals 150, 160, as described further below. Scheduler 116 then receives the expected SIRs from receiver 140 and/or SIR processor 118 and schedules one of the plurality of mobile terminals 150, 160 based on the provided SIRs.
Each mobile terminal 150, 160 includes a transceiver 152, an antenna 154, a measurement circuit 156, and an SIR processor 118. Each transceiver includes a transmitter 157 for transmitting signals to the base station 110 via antenna 154 and a receiver 158 for receiving signals from the base station 110 via antenna 154. According to the present invention, measurement circuit 156 in scheduled mobile terminal 150 despreads the corresponding traffic channel, estimates the traffic channel SIR, and sends the estimated SIR and/or a representation or mapping of the estimated SIR, e.g., a CQI, to base station 110 for processing at the scheduler 116. A CQI is typically a 5-bit number that corresponds to predetermined SIR values. Because the scheduled mobile terminal 150 despreads the traffic channel, it is able to estimate the gain vector h0 and the impairment covariance matrix R0, and thus the SIR on the traffic channel. The gain vector ho has exactly the same form as hn in Equation 6, except that hnl is replaced by h0l, i.e., the tap gains of the effective traffic channel heff,0(t). These tap gains are calculated by estimating the tap gains of each of the channels {g0m(t)}m=1M using the pilots, and then using the equation given by Equation 2 to calculate the effective channel (in the frequency domain). The impairment covariance matrix R0 may be calculated by performing a time average of the despread traffic channel. The despread vector is y0(i). The estimated impairment covariance matrix is therefore given by:
{circumflex over (R)}z,0=<y0(i)y0H(i)>−√{square root over (αsET)}ĥ0ĥ0H, (Eq. 21)
where ĥ0 is the estimated gain vector and <·> signifies a time average. The SIR estimate is then given by:
where R0=IoRz,0. Therefore, while the scheduled mobile terminal 150 is scheduled, SIR processor 118 may estimate the SIR using the despread traffic channel. Alternatively, the SIR processor 118 may treat the scheduled mobile terminal 150 as a non-scheduled mobile terminal 160 and estimate the SIR for the scheduled mobile terminal according to the embodiments discussed further below.
Because the non-scheduled mobile terminal 160 does not have knowledge of the multi-path propagation channel associated with the scheduled mobile terminal 150, the SIR measured by measurement circuit 156 in the non-scheduled mobile terminal 160 does not correspond to the expected SIR of a future traffic channel transmission. Therefore, in order to generate the expected SIR of a non-scheduled mobile terminal 160, the SIR processor 118 computes the expected SIR based on a hypothesized traffic channel that would have resulted if the non-scheduled mobile terminal 160 was scheduled, as described further below. The computed SIR is then provided to scheduler 116.
According to the present invention, the SIR processor 118 in non-scheduled mobile terminal 160 may compute the expected SIR and then transmit the computed SIR to the base station 110 for further processing in scheduler 116. Alternatively, the non-scheduled mobile terminal 160 may transmit the SIR variables generated by measurement circuit 156 to the base station 110 for further processing in the base station SIR processor 118. The base station SIR processor 118 then computes the expected SIR and forwards the computed SIR to the scheduler 116 for further processing, as discussed above.
In a first exemplary embodiment of the present invention, the SIR processor 118 in either the non-scheduled mobile terminal 160 or in base station 110 computes an expected SIR of a hypothesized traffic channel associated with the non-scheduled mobile terminal 160 that would have resulted if the non-scheduled mobile terminal 160 was scheduled by applying a correction factor to a measured pilot SIR associated with the non-scheduled mobile terminal 160. The correction factor compensates for the channel mismatch between the pilot channel and the effective traffic channel that would exist if the non-scheduled mobile terminal 160 was scheduled. In general, the measurement circuit 156 of the non-scheduled mobile terminal 160 measures SIR variables, i.e., pilot channel SIR (SIRp), and provides the SIR variables to the SIR processor 118. Measurement circuit 156 may also measure a delay spread θd (another SIR variable) corresponding to the pilot channel signals. Alternatively, a nominal delay spread θd may be stored in memory for use by SIR processor 118. SIR processor 118 then determines the correction factor, φn, based on the measured SIR variables, as discussed further below, and applies the correction factor φn and optionally a power scalar αs/αp to the measured pilot SIRp to compensate for the channel and power mismatch between the pilot channel and the hypothesized traffic channel.
In an exemplary embodiment, as shown in
Further, different non-traffic power ratios (αd1, αd2, . . . αdj) correspond to different sets of projected pilot SIRs and correction factors φn. In other words, as shown in
Further still, the set of curves illustrated in
Referring now to
The above-described embodiment compensates for the SIR mismatch in a non-scheduled mobile terminal 160 by applying a correction factor to a measured pilot SIR of a non-scheduled mobile terminal 160.
If the mismatch is to be corrected at the base station 110, non-scheduled mobile terminal 160 transmits the SIR variables to the SIR processor 118 in base station 110 (block 220). Using the pilot SIR received from the non-scheduled mobile terminal 160, delay spread θd (either measured or nominal) and/or power ratio αd (either measured or nominal) (block 222), SIR calculator 120 in base station 110 selects the correction factor φn from the look-up table stored in memory 122 (block 224) and applies the correction factor φn to the measured pilot SIR (block 226) to generate the expected SIR (SIRn).
This process is repeated for each non-scheduled mobile terminal 160 in the wireless system. Further, the scheduled mobile terminal 150 provides an SIR corresponding to the scheduled mobile terminal 150 to the base station 110. Scheduler 116 then evaluates the SIRs (block 230) and schedules one of the mobile terminals 150, 160 based on the SIRs (block 232).
A second embodiment of the present invention compensates for the above described effective channel mismatch by hypothesizing the effective traffic channel of the non-scheduled mobile terminal 160 that would have resulted if the non-scheduled mobile terminal 160 was scheduled, and directly computing the expected SIR of the non-scheduled mobile terminal 160 based on the hypothesized effective traffic channel using Equation 12. As discussed above, non-scheduled mobile terminal 160 has knowledge of the effective channel and can compute the pre-filters 22, 24 that would be used if non-scheduled mobile terminal 160 was scheduled, and therefore has knowledge of the channel gain vector hn and the noise covariance matrix Rn. Because non-scheduled mobile terminal 160 also can assume some known value of the total received signal energy ET and has access to power ratio αd, the non-scheduled mobile terminal 160 has access to all of the variables necessary to compute the SIR using Equation 12 except for an estimate of the underlying noise level Io. Therefore, an exemplary SIR processor 118 for the second embodiment further includes means for estimating the underlying noise level Io, in addition to SIR calculator 120.
As shown in
Noise estimator 180 may generate the noise estimate Îo, according to any known method. For example, the noise estimate Îo may be generated according to the method disclosed in commonly assigned U.S. patent application Ser. No. 09/660,050, entitled “Apparatus for and Method of Adapting a Radio Receiver Using Control Functions” and filed 12 Sep. 2000, which is incorporated herein by reference.
Alternatively, noise estimator 180 may generate the noise estimate Îo based on an estimate of an interference noise level, a combination of the interference I and the underlying noise Io, over different frames of a received signal. In this embodiment, shown in
The above-described embodiment calculates the SIR based on a noise estimate Îo. While practical implementations of the second embodiment may perform this calculation at the non-scheduled mobile terminal 160, those skilled in the art will appreciate that the base station 110 may also calculate the expected SIR of the non-scheduled mobile terminal 160 provided that the non-scheduled mobile terminal 160 supplies the base station 110 with the necessary SIR variables.
The second exemplary embodiment of the present invention compensates for the SIR mismatch by directly computing an expected SIR for the non-scheduled mobile terminal 160, based on a noise estimate Îo as if the non-scheduled mobile terminal 160 was scheduled.
Instead of estimating Io, the SIR processor 118 in each non-scheduled mobile terminal 160 may directly compute an expected SIR according to Equation 12 using a preset noise level predetermined by the base station 110 and stored in memory 122, as shown in
The above-described invention provides an improved method and apparatus for estimating an expected SIR for a non-scheduled mobile terminal 160, and therefore, provides an improved method and apparatus for scheduling mobile terminals 150, 160 in a wireless communication system 100. While the previous discussions focused on wireless systems that use the HS-DSCH mode of a W-CDMA system, those skilled in the art will appreciate that the above described method and apparatus is applicable to any wireless communication system that pre-filters traffic channel signals separately from pilot channel signals. As such, the above-described problem is present in any wireless communication system where the effective traffic channel differs from the effective pilot channel due to the pre-filters associated with the traffic channel signals.
The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
Molnar, Karl James, Krasny, Leonid, Grant, Stephen James
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